Cellulose fibres are mainly formed from cellulose, and their chemical reactivity is determined by the presence of numerous glucan units linked together in 1,4 position as shown in FIG. 12,6.
The chemical structure of cellulose fibres shown schematically in FIG. 1 shows that the hydroxyl groups 2-OH, 3-OH and 6-OH present on each glucan unit are available for chemical reactions similar to the reactions in which primary, secondary and tertiary alcohols can participate2,6.
Cellulose fibres are associated with each other through the formation of intermolecular hydrogen bonds (shown schematically in FIG. 2); such links give rise to structures known as microfibrils that are organized into macrofibrils, which in turn are organized into fibres.
Chemicals with access to the internal pores of the fibre will find that many potentially reactive sites are not available due to their involvement in the aforementioned hydrogen bridges.
The reactivity of the hydroxyl groups of the cellulose fibres, as determined by chemical kinetics measurement, is in the order 2-OH>3-6-OH>>OH2,6.
Because cellulose fibres are widely used in the production of soft furnishings for both private and public spaces (such as theatres, cinemas, conference rooms, etc.), research has focused on improving the anti-flame properties of cellulose fibres.
Combustion takes place when there is:                a fuel;        an comburent, commonly oxygen in the air; and        a heat source capable of promoting physical and chemical changes on the comburent such as to initiate the combustion of the fuel.        
From this moment, the phenomenon of combustion is self-powered by the heat produced.
An important step in the combustion of cellulose fibres is the initial formation of the depolymerisation product levoglucosan, as illustrated in FIG. 3.
Levoglucosan is the precursor of volatile inflammables, which are the main contributors to the speed of propagation of cellulose combustion.
Reducing the formation of levoglucosan reduces the amount of volatile combustibles and therefore the flammability of the cellulose fibres.
Fibrous materials can be inorganic, thus incombustible (asbestos, glass, ceramics), or organic (cellulose, wool, artificial or synthetic fibres).
The combustibility of fibrous materials of organic origin is measured, for example, in terms of the minimum quantity of oxygen required to burn a fibre. In this sense, the Oxygen Index (OI) or Limiting Oxygen Index (L.O.I.) is the minimum percent concentration of oxygen (vol) at which combustion continues for 3 minutes after ignition of the tip of the specimen with a free flame which is then removed.
A material with an OI>21, corresponding to the oxygen content of air should not give self-sustaining combustion, even if triggered. However, for prudence dictated by differences between the test conditions and those of a fire, materials are indicatively considered to be flame retardant materials when O.I. is >25. Table 1 lists the reaction to fire and values of O.I. for the main polymers used in textiles.
TABLE 1Reaction to fireMaterialL.O.I. (%)Fibres that ignitePolypropylene18easilyAcrylic19Cotton20Polyamide22Polyester22Fibres with flame Wool25retardantPolypropylene FR27characteristics293131-34Heat resistantAramid29-34fibresPolyamide-imide30-32Cross-linked polyacrylate45Polybenzene-imidazole48Preoxidised Acrylics50
The first group includes both natural and artificial fibres that are easily inflammable, characterized by an L.O.I. around 18 (cotton, acrylic, polypropylene, cellulose fibres). Other synthetic fibres have a L.O.I. around 22 (polyamide, polyester), and provide behaviour acceptable only in less critical applications (flooring, wall coverings, etc.). In this group, wool with a value of around 25 is the only fibre that can almost be defined as a natural flame retardant.
The second group includes fibres treated to impart anti-flame properties, characterized by L.O.I. values comprised between 28 and 31. These are the fibres most used for the production of textiles, intended for use in all areas at risk of fire.
To reduce the combustion capability of cellulose fibres, recourse has been taken to various procedures that chemically modify the fibres themselves.
Procedures currently known to reduce the combustibility of cellulose fibres are divided into two categories:                application—by physical processes—on the fibrous material of intumescents and barriers that reduce the contact with the comburent, and/or        application—through chemical processes—on the fibrous material of inhibitors that interfere with the combination of volatile fuels (levoglucosan) with the comburent.        
In the following, the term “physical treatment” of a cellulose substrate means a process that does not involve a chemical modification of the cellulose fibres, but rather a simple application on the surface of the cellulose substrate of substances capable of imparting anti-flame properties.
The term “chemical treatment” means a process that involves a modification of the chemical structure of cellulose fibres to impart anti-flame properties to the cellulose substrate treated.
In the following, further details will be provided concerning the various procedures known in the art for protecting cellulose fibres from fire.
A method for protecting cellulose fibres from fire provides for the application by means of coating on the surface of the textile material itself of “ceramic materials” that—by delaying the transmission of heat to the fibrous polymer—decrease and control pyrolysis and ignition.
A different method for imparting anti-flame properties to cellulose fibres envisions application to the fibres by means of coating of aluminium hydroxide and magnesium that—by absorbing heat and decomposing via strongly endothermic reactions—prevent attainment of the ignition temperature of the fibre, thus avoiding combustion or making it more controllable.
Until now, the use of boron salts is known; applied by coating on and/or impregnation of the cellulose fibres, in the presence of heat (such as that generated by a heat source) these emit water vapour and produce a vitreous “foam” with scarce heat conduction capacity around the fibre.
The application of halogen-containing compounds and/or antimony-based compound by means of coating is also known.
In the following, information will be provided about methods for the chemical treatment of a cellulose substrate that involve modification of the chemical structure of the cellulose constituting the cellulose fibres of the substrate to impart anti-flame properties to the substrate.
A method for imparting anti-flame properties to cellulose fibres involves applying to the fibres, by means of a chemical treatment, compounds that reduce the formation of volatile flammable compounds favouring the formation of a carbonaceous residue. Carbon residue is much less flammable than volatile organics, and its oxidation (combustion) is slower and in always localized, thus making the propagation of the combustion controllable.
This effect can be obtained, for example, promoting the dehydration of the cellulose to carbon at a temperature lower than that of the usual pyrolysis of the fibre.
The thermal dehydration of cellulose with the formation of carbonaceous residue is mainly promoted by acids. Chemicals that can provide acid in the first stage of pyrolysis constitute major fireproofing products for cellulose fibres. The presence of nitrogen also makes an important contribution to the mechanism of fireproofing. A significant reduction of the formation of levoglucosan is obtained by means of esterification of the primary hydroxyl in position C6 of the cellulose by phosphoric acid derivatives.
It is also possible to interfere with the combustion of the cellulose fibres by promoting the formation of incombustible gas, which is mainly realized by application to the fibre (through a chemical reaction) of nitrogen compounds or ammonium base or of compounds containing halogens. At the pyrolysis temperature, these produce non-combustible gases such as ammonia, water, CO2, halo acids, etc. that simultaneously dilute the concentrations of the oxidiser (oxygen in the air) and the flammable gases from the pyrolysis of cellulose fibre.
There are also other methods to improve the flame resistance of cellulose fibres.
Methods are known that provide treatments of the cellulose fibres employing salts or partially salified acids of phosphorus, derivatives based on N-methylamides of phosphines and phosphine oxides, vinyl phosphonates. However, these methods are not satisfactory because they create chemically modified cellulose fibres with a low flame resistance.
Other phosphorus-based compounds have been identified, such as diethyl {3-[(hydroxymethyl)amino]-3-oxypropyl}phosphonate, to improve the flame resistance of cellulose. The application of these compounds provides cellulose substrates that can pass the vertical flame test, but unfortunately makes the hand of the finished products very hard and rough, not to mention the production of formaldehyde during the process that affects worker safety in the workplace.
A different procedure for imparting anti-flame properties to cellulose fibres, and currently the most used, involves the polymerization of the monomer tetrakis(hydroxymethyl)phosphonium chloride with ammonia in anhydrous phase directly on the cellulose substrate. Apart from the hazard of the reaction, the formation of such polymers involves a drastic change of the appearance of the finished textile substrate, in particular making the hand of the fabric stiff and rough.
In view of the insufficient anti-flame properties of cellulose fibres obtainable with the previously described procedures, methods for sulphation and phosphorylation of fibres had already been developed in the 50s.
The known reactions that take place in the process of sulphation of cellulose are the following4,5,7:Cell-OH+NH2SO2ONH4→Cell-OSO2NH2+NH3+H2O2 Cell-OH+NH2SO2ONH4→Cell-OSO2O-Cell+2 NH3+H2OCell-OH+NH2SO2ONH4→Cell-OSO2ONH4+NH3 
The known reaction that takes place during the process of phosphorylation of cellulose is9:Cell-OH+H3PO4→Cell-OPO3H2+H2O
Typical formulations for conducting the cellulose sulphation reaction consist of solutions based on aqueous ammonium sulphamate and urea or its derivatives.
Typical formulations for conducting the cellulose phosphorylation reaction consist essentially of aqueous solutions based on phosphoric acid and/or its mono and dibasic salts and/or phosphoramide and its derivatives.
The method of sulphation and phosphorylation of a cellulose substrate is achieved by immersion through pressing of the substrate in a solution containing the sulphation compounds and subsequently in a solution containing the phosphorylation compounds, drying the substrate, allowing the sulphation and phosphorylation reactions to go to completion at temperatures above 180° C.4,5,7 for a several minutes; reaction by-products are removed by washing the treated substrate.
However, such procedure has numerous disadvantages.
The first and main disadvantage is the loss of resistance of the treated substrate.
From a chemical point of view the strong acidity developed in the reaction phase by phosphoric acids and by ammonium sulphamate and the high temperatures needed for completion of the reactions cause degradation of the 1,4 glucosidic bonds of cellulose, with a substantial drop in the molecular weight of the cellulose polymer and subsequent loss of up to 70% of the tensile strength of the substrate4,5,7.
The second disadvantage of the sulphation and phosphorylation method is determined by the different kinetics of the two reactions.
The sulphation reaction is very slow when applied to solid substrates such as cellulose, while phosphorylation on the contrary is very fast; it follows that the when sufficient sulphation of the substrate has been obtained, the excessive phosphorylation damages it irreparably. Also, it is important to note that the excessive presence of phosphorus does not improve the anti-flame properties of the substrate.
Therefore, it is essential to proceed in two distinct reaction steps with a very significant increase in the production costs.
A separate disadvantage is related to the lack of efficacy of the procedure in conferring anti-flame properties to the cellulose substrate: the mechanism of phosphorylation preferably employs phosphoryl acids or their salts.
The yield of this reaction is very high, but when the substrate is subjected to washing in water, calcium and magnesium ions present in the water bind to the unreacted hydroxyl groups of phosphoryl groups linked to the cellulose substrate to form insoluble salts that inhibit the anti-flame power of the substrate, as illustrated in FIG. 4.
To eliminate the disadvantages associated with phosphorylation reactions using phosphoric acid, the use of nitrogen-containing resins such as derivatives of dimethylolethylene-urea or dimethylol dihydroxyethylene-urea during the phosphorylation and/or sulphation reactions has been proposed.
However, such alternative has problems both in the processing phase and with the finished product: the release of free formaldehyde during the reaction causes safety problems for the operators, also formaldehyde binds to the cellulose substrate by covalent bonds determining problems with toxicity and carcinogenicity of the substrate itself.
In addition, an environmental problem is associated with the sulphation and phosphorylation method. The sulphation reaction requires urea (or its derivatives), which—at the moment when it reaches its melting point around 130-135° C.—creates a dense, white mist4,5,7 inside the reactor (oven) and its sublimation gives rise to accumulations on the reactor walls.
The sulphation reaction releases also ammonia gas as a by-product with the need to neutralize the fumes resulting in a significant economic impact on the industrial process.
The method of sulphation and phosphorylation implemented using phosphoric acid was commercially abandoned in the 70s in view of the numerous disadvantages discussed above.